Heavy ions are being raised to treat some cancer patients. Some data is available on toxicity in a few normal tissues but only for early damage. Also, a potentiation of injury has been observed with fractionation of the neon beam in contrast to the well-known sparing effect obtained with fractionation of X-rays. This study will determine the repair or potentiation of both early and late damage in mouse lungs after fractionated doses of neon ions.